A lithium ion secondary battery is expected to be applied in a wide field such as an electronic device, on-vehicle use, or stationary use.
In general, as a positive electrode active material for a lithium ion battery, layered rock salt type LiCoO2, spinel type LiMn2O4, or the like has been partially put into practice. When one electron reaction is assumed, the layered rock salt type LiCoO2 has a theoretical electrical capacity of 273 mAh/g. The spinel type LiMn2O4 has a theoretical electrical capacity of 148 mAh/g.
NPL 1 discloses a crystal structure of Li2MnO2 having a layered structure. The space group of Li2MnO2 having a layered structure is trigonal P-3m1 (No. 164). The space group of a layered rock salt type crystal structure is R-3m (No. 160), and the space group of a spinel type crystal structure is Fd-3m (No. 227). Therefore, the crystal structure of Li2MnO2 having a layered structure is different from the structures of these positive electrode active materials.
PTL 1 describes use of Li2MnO2 having a layered structure as a positive electrode active material. When one electron reaction is assumed, Li2MnO2 having a layered structure has a theoretical electrical capacity of 530 mAh/g, larger than the theoretical electrical capacity of the layered rock salt type LiCoO2 or the spinel type LiMn2O4.
PTL 2 describes that the open-circuit potential is less than 2 V in charge/discharge of Li2MnO2 having a layered structure.